Do not leave me alone with them!

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Just like with people, opposites attract and likes repel in relationships between ions. If nature builds material from ions – be it the salt crust at the edge of the Dead Sea or small crystals formed at the bottom of a solution – negative and positive ions do it together. What happens if we remove one type of ions from the equation and bring more and more ions of the same polarity together? The material should explode due to Coulomb repulsion! However, the situation is very different if we accumulate ions over a thin insulating layer on top of a conductive surface. Like in a very thin capacitor, such a surface builds up an opposite charge to that of the ions to achieve a counterbalance. The ions have a partner, but it is like a relationship through a fence if we do not allow the ions to neutralize on the surface. To explore this scenario, we accumulated a large amount of anions – very stable in structure and charge state – on a surface. Will the anions be happy when surrounded by other anions and separated from their partners by a fence? Probably not! Instead of ripping the material apart or breaking the fence (neutralization), nature reacts to this man-made situation in a surprising and constructive way by bringing molecules from the surroundings to the rescue.

A thin condensed-phase layer of material formed in this process. At first it appears as a dull brown spot in vacuum. When exposed to air, the layer breaks up into droplets showing a liquid-like behavior. The resulting drops on the surface scatter light, creating colorful displays which are dependent on the deposited ions, the accumulated molecules and the surface. The complex and sensitive evolution of the material can be traced back to its frustrated nature. The material is hungry for anything that will stabilize it – it wants more positive ions! This frustration may be used to sensitively manipulate the material and its self-organization on surfaces.

Two different morphologies of layers which were obtained by deposition of similar anions differing only in one molecular parameter (left: [B­12Cl12]2-, right: [B12I12]2-).

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